1. Field of the Invention
This invention relates to a method of adjusting a scale factor when a radiation image stored in a stimulable phosphor sheet is read out. This invention particularly relates to a method of adjusting the latitude in final read-out by detecting in advance the image input level of a radiation image by preliminary read-out conducted prior to the final read-out from a stimulable phosphor sheet carrying the radiation image stored therein.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image recording and reproducing system using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored in the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted from the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor sheet or the photodetector, a change in radiation dose according to the condition of the object, or a fluctuation in the radiation transmittance according to the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low. Further, it is possible to obtain a radiation image having a high image quality of high contrast, high sharpness and low noise, and the like, by converting the light emitted from the stimulable phosphor sheet into an electric signal, and processing the electric signal as desired. Particularly, when the radiation image is used for medical diagnosis, it is possible to obtain a radiation image processed in the manner most suitable for a particular portion of the human body such as the heart, the chest or the like, thereby realizing an improvement in diagnostic efficiency and accuracy.
However, in order to eliminate various influences caused by the fluctuation of radiographic exposure conditions and/or to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate such image input conditions of the radiation image stored in the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the body (e.g. the chest or the abdomen) or the radiographic method used, such as plain image or contrasted image radiographing, before reproducing the radiation image to a visible image, and then to appropriately adjust the read-out conditions such as read-out gain or scale factor. Then, the image signal read out based on the adjusted read-out conditions, is appropriately processed. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically.
Investigation of the image input information may be conducted prior to the visible image reproduction by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the method, a read-out operation for detecting the image input information of a radiation image stored in a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a subsequent read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. In the final read-out, the read-out conditions such as the read-out gain and/or the scale factor are appropriately adjusted on the basis of the image input information obtained by the preliminary read-out. In this method, since the image input conditions and the image input pattern of a radiation image stored in the stimulable phosphor sheet can be investigated in advance, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy by adjusting the read-out gain and/or the scale factor based on the detected image input information in the manner most suitable for the image input pattern without using a read-out system having a wide dynamic range. Then, when required, the image signal is further processed to obtain an image of higher quality.
When the aforesaid method is applied, levels of signals detected by the preliminary read-out over the whole surface of the stimulable phosphor sheet are stored in a memory, and a histogram is created by calculating the stored signal levels by use of a signal processing apparatus. From the histogram, the maximum stimulated emission intensity (Smax) and the minimum stimulated emission intensity (Smin) necessary for viewing, particularly for diagnostic purposes are determined. Then, the scale factor (i.e. latitude) in the final read-out is determined on the basis of Smax and Smin.
In the case where the radiation image recording and reproducing system is used for medical diagnosis, portions of the human body not related to diagnosis should not be exposed to radiation since the radiation is harmful to the human body. Further, when the human body portions not related to diagnosis are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in many cases, the radiation exposure field should be limited when a radiation image is recorded. Normally, when the radiation exposure field is limited, radiation scattered by the object within the radiation exposure field passes outside of the radiation exposure field. The scattered radiation is absorbed and stored in the stimulable phosphor sheet, which exhibits high sensitivity. However, when the scale factor is determined by the aforesaid conventional method, it is not always possible to discriminate between the energy of the scattered radiation and the radiation energy of the image portion within the radiation exposure field, and the minimum stimulated emission intensity caused by the scattered radiation is detected as Smin. Normally, the minimum stimulated emission intensity of the scattered radiation is very much smaller than that at the image portion within the radiation exposure field. Therefore, when the minimum stimulated emission intensity caused by the scattered radiation is detected as Smin, since the signal caused by the scattered radiation not related to diagnosis is taken within a low density range in the final read-out, the density of the image of the portion related to diagnosis becomes too high. As a result, the image contrast decreases and it becomes difficult to make an accurate diagnosis.